A large number of organisms are present in sea water or fresh water, and these organisms show pathogenicity or are adhered to surfaces of submerged structures, causing various problems. For example, when organisms are adhered, a propulsion resistance is increased in marine vessels, a heat exchange efficiency is decreased in cooling pipelines used in thermal electric power plants, and large organisms which are adhered to insides of cooling pipelines and grown are removed to clog the cooling pipelines. Further, a large amount of water is used in food processing or production of drinking water and toiletries, and is supplied through feed water pipes. When microorganisms are adhered to insides of feed water pipes and grown, the microorganisms are incorporated into products, inviting serious defects in the quality of products.
Generally, a mechanism of adhering organisms to water contact surfaces of submerged structures is as follows. First, adhesive Gram-negative bacteria are adsorbed on surfaces of submerged structures to secrete large amounts of slime-like materials derived from lipids. Further, Gram-negative bacteria are gathered on this slime layer, and grown to form a biofilm. And large organisms such as algae, shellfish, barnacles and the like are adhered to this biofilm layer, and the large organisms adhered thereto are proliferated and grown, finally covering up the water contact surfaces of the submerged structures.
In recent years, as a method for preventing stains due to organisms adhered to water contact surfaces of submerged structures such as marine vessels, fishing nets, feed water pipes and the like, a method for electrochemically controlling organisms adhered to submerged structures without generating harmful substances such as chlorine and the like has been proposed. This electrochemical controlling method is performed such that when a potential above a predetermined potential at which a direct reaction with microorganisms has been identified is applied to the microorganisms, coenzyme A, one of the redox substances in microorganisms is reversibly oxidized, making it possible to kill microorganisms upon inducing the decrease in a breathing activity of microorganism and a permeation barrier of membranes of microorganisms (Japanese Patent Publication No. 6-91821/1994). That is, a method for preventing adhesion of large organisms by electrochemically controlling adhesion of Gram-negative bacteria is described.
As a stain prevention apparatus of for a submerged structure in contact with sea water or fresh water utilizing the above-described electrochemical controlling method, an apparatus is used comprising a submerged structure of which the water contact surface is coated with a conductive resin layer such as a conductive rubber, a conductive coating film or the like, a counter electrode disposed opposite so as not to come in contact with the conductive resin layer and a power supply for passing a direct current through the conductive resin layer and the counter electrode.
The conductive resin layer in the above-described stain prevention apparatus is formed by dispersing conductive fine particles of carbon black, graphite or the like into a synthetic resin. When a potential is applied to the conductive resin layer containing such conductive fine particles in sea water, sea water is electrolyzed to generate harmful chlorine. There is a fear that the chlorine gas generated might expedite corrosion of a submerged structure formed of a metal, inhibit growth of useful cultured fish and further influence ecosystem. Accordingly, when a conductive resin is used in an electrode, the potential control is conducted using a reference electrode for applying an accurate potential at which sea water is not electrolyzed (Japanese Patent Laid-Open No. 4-78482/1992 and Japanese Patent Laid-Open No. 4-313379/1992).
When an area of a counter electrode is smaller than an area of a conductive resin layer in the potential control using this reference electrode, the potential of the counter electrode is increased, and harmful chlorine is generated by electrolysis of sea water. Thus, it is required that the area of the counter electrode is widened and that the counter electrode is located opposite the conductive resin layer at a fixed distance between the counter electrode and the conductive resin layer. However, when a counter electrode of a large area is located opposite the hull in marine vessels, there occur the other problems such as the increase in the propulsion resistance due to the counter electrode, the damage of the counter electrode by contacting the counter electrode with a pier in the stop at a port, further the damage of the hull and the like. Moreover, in the cooling water intake pipeline, the volume inside the pipeline is limited. When a counter electrode of a large area is disposed, a water supply capacity is decreased, and further it is impossible to dispose a counter electrode in some type or structure of a submerged structure. Accordingly, a novel electrochemical stain prevention apparatus of a submersed structure which can solve the above-mentioned problems is in demand.